I only buy new if I am looking for a specific book, and it has proven difficult to find. Otherwise, book prices are so outrageous that I have to buy used to justify the cost.
I agree. Were you to print a copy of the book, that would be another story.
Sorry for the other long reply, but intellectual property has been a topic with me lately...
With exceptions, most of the time when books get sold as used the publisher has quit producing them. No royalties are lost by the author because no more copies are being produced.
Often the only way to get much sought after titles is to buy them used. And that is especially true for first editions or autographed copies, which can be highly prized and much sought after.
Once a person relinquishes his ownership of the book, and has not violated any other contractual obligations for the book, (I'm not aware of any such obligations not to resell the book, as opposed to re-selling some Microsoft software products, for example) there is no problem with buying a used book. The problem is when multiple copies are being shared simultaneously, i.e. burning a music CD over and over and handing it over to friends while you still groove to the original--that's a no-no.
Personally, I buy used books. I'm a big fan of used book stores, but I'm a bigger fan of the public library system.